Free-space optical interconnects (FSOIs) have been proposed as a means of data transfer between system modules. Examples of system modules are chips, circuit boards, computing modules and equipment enclosures containing the aforementioned. Techniques commonly used are passive and active mechanical alignment mechanisms. Many systems contain serviceable modules that may be installed and removed from the system which add to the complexity and cost of mechanical alignment for FSOIs. Often, these systems employ backplanes or mid-planes. Backplanes are often characterized as having slots on one side of the printed circuit board to accept connections, whereas mid-planes are often characterized as having slots on both sides of the printed circuit board.
Without alignment, light power detected at the receiver may be reduced and/or optical crosstalk may increase which results in the degradation of the data carrying capacity of the FSOIs. Misalignment is characterized as the displacement from the nominal alignment. Displacements can be horizontal, vertical, longitudinal, angular or any combination of the aforementioned. The sources and range of misalignment are numerous and varied.
Misalignments range from thermal expansion/contraction effects to equipment shifting due to earthquakes. Other sources include manufacturing imperfections, vibrations, installation misalignment and floor sagging to name a few. The challenges of establishing and maintaining alignment are further compounded as the number of optical interconnects is increased.
Many alignment techniques are used to compensate a limited range of misalignment. These include precise manufacturing tolerances, design geometry, out-of-band misalignment monitoring, adjustment and rerouting data channels to redundant elements.
Optical beam divergence of a transmit light emitting device and diffraction effects along a path severely limit the distances in which a receive photo-detector can detect sufficient light so that data can be reliably extracted. Beam divergence and diffraction affect the distance over which reliable communication data can flow. In addition, beam divergence, diffraction and other sources can cause undesirable crosstalk in FSOIs with multiple interconnects that are spaced close to one another. Further degradation can occur by stray light which may find its way to the receive photo-detectors.
FSOIs may lose some or all of the ability to transfer data when line-of-sight obstructions are present. Obstructions include, but are not limited to cables, dust and personnel interference. This is exacerbated when multiple FSOIs are closely packed with one another. The ability to transfer data is also affected by component degradation and failures including, but not limited to loss of transmit light emitter luminosity and loss of receive photo-detector sensitivity.
Therefore, there is a need for development of FSOIs tolerant to a range of displacements from the nominal alignment without the need for mechanical alignment mechanisms, achieve data communication over longer line-of-sight distances, achieve partial obstruction immunity, increased isolation from crosstalk and stray light, increased tolerance to component failures and degradation, the methods of operation thereof which would provide resilient and reliable data transfer.
A blade server and data processing module are examples of a computer module. The rearward end of the modules often includes connectors that mate with backplane connectors within the enclosure or chassis when the removable modules are inserted into the enclosure or chassis. Data communication between modules is often through passive backplane or mid-plane connections. The connections may be metal-based, optical-based or a combination of either of the aforementioned. The connections may be point-to-point or shared which bounds the data carrying capacity of the backplane or mid-plane to the size of the backplane or mid-plane and the number of connections provided by the connectors on the backplane or mid-plane. An increased requirement in the number of modules or data transfer between modules necessitates a larger backplane or mid-plane to accommodate a larger number of connections which has practical limits.
Therefore, there is a further need for development of an expandable backplane or mid-plane system that is not bounded by the practical limits of a single backplane or mid-plane such that as the number of modules increases, there is a corresponding increase in the number of connections between modules contained in the system and a corresponding increase in the data carrying capacity of the system.